1. Field of the Invention
Embodiments of the present invention relate to a control apparatus intended for a hybrid vehicle including an engine and a motor and, more particularly, to a control apparatus that executes control for starting the engine at a time when the hybrid vehicle is traveling by using only the power of the motor.
2. Description of Related Art
Japanese Patent Application Publication No. 2011-73564 (JP 2011-73564 A) describes an apparatus that starts an engine in so-called EV mode (electric traveling mode) in which a hybrid vehicle is traveling by using only the driving force of a motor. A hybrid vehicle is described as an example. In the hybrid vehicle, an engine and a first motor are coupled to a power split mechanism formed of a differential mechanism, driving force output from the engine is output from the power split mechanism to an output member, and the first motor is caused to function as a generator by driving the first motor with the use of the engine. In the hybrid vehicle, a second motor is coupled to the output member, the second motor is driven as a motor by using electric power generated by the first motor.
In the hybrid vehicle described in JP 2011-73564 A, when the second motor is caused to function as a motor by using the electric power of an electrical storage device in a state where the operation of the engine is stopped, the hybrid vehicle is set to a mode in which the hybrid vehicle travels by using electric power (EV mode) or functions an electric vehicle. As for a driving force that is obtained in a state where only the motor is used as a driving force source, an outputtable driving force becomes insufficient for a required driving force where a driver performs acceleration operation. In such a case, the engine is started by rotating the first motor, coupled to the engine via the power split mechanism, in a positive direction (a rotation direction of the engine during steady operation). That is, when the first motor rotates in the positive direction to output torque, positive torque acts on the rotating element, to which the engine is coupled, in the power split mechanism, so it is possible to raise the rotation speed of the engine. In such a case, negative torque (torque in a direction to stop rotation) acts on the rotating element to which the output member is coupled. Therefore, the driving torque of the hybrid vehicle is reduced by the use of the output torque of the first motor at a startup of the engine. Therefore, in the thus configured hybrid vehicle, when the engine is started at the time when the hybrid vehicle is traveling in EV mode, the output torque of the second motor is increased for the purpose of suppressing a decrease in driving force.
In the apparatus described in JP 2011-73564 A, a torque value obtained by subtracting a reaction torque resulting from a startup of the engine and a margin, which increases with an increase in the output limitation of the battery and reduces with an increase in the vehicle speed, from a rated torque of the motor that is used for traveling is set as a threshold, and the engine is started when the required torque exceeds the threshold.
Japanese Patent Application Publication No. 2013-86704 (JP 2013-86704 A) describes an apparatus configured to cause a vehicle to travel in EV mode by causing the above-described second motor to function as a motor by using the electric power of an electrical storage device, and if a required power exceeds a threshold, an engine is started. The threshold in the apparatus described in JP 2013-86704 A is set to a power obtained by subtracting a power, which is required to start the engine, and a predetermined margin from an output upper limit value limited in the electrical storage device.